... a British- (cockney-) accented version of "Not half"??
Yes that's it, but I'm not entirely sure it's specifically London cockney (though cockneys certainly like using it).
It does indeed mean "Not half".
So if someone asks you if you like the Stones and you say "Not 'arf", it means you absolutely love the Stones.
Where I come from you could say, for example, "He isn't 'arf daft", meaning he's completely stupid.
It cropped up in a BBC comedy series from a few years ago called "It ain't half hot mum." meaning it's very hot indeed (the show took place in India).
It's worth watching for the different accents on display - scottish, cockney, urbane middle-class, upper-class etc.
The sergeant major is excellent.